The present invention relates to a thermal copier for reproducing an image printed on a document on a thermosensitive sheet overlapping the document by illuminating the document and sheet by thermal rays for which the sheet is transparent.
A thermal copying procedure uses a thermosensitive sheet and transports it to a predetermined exposing position together with a document carrying a desired image thereon. In the exposing position, the document is illuminated by ultraviolet rays that pass through the thermosensitive sheet. Then, the image of the document absorbs the thermal rays and is heated thereby, and the heat is transferred to the photosensitive sheet. As a result, a thermosensitive coloring agent provided on the sheet is heated to color in a pattern identical with the image of the document. To cause the coloring agent on the sheet to color due to the heat transferred from the document, it is necessary that the document and the sheet be pressed against each other at least in the exposing position. To meet this requirement, a copier adopting such a procedure has customarily been provided with a transport belt which transports a document and a thermosensitive sheet laid one upon the other to an exposing position and presses them against a platen there. The transport belt is heat-resistive and transparent for ultraviolet rays. In the exposing position, a lamp illuminates the document through the belt and thermosensitive sheet to heat an image printed on the document. The belt is usually implemented as an endless belt and driven by rollers. The simplest way to produce the endless belt is cutting a transparent film in an elongate strip configuration and then joining opposite ends of the strip to form a loop. However, the belt so produced is not usable in practice since the seam where the opposite ends are joined together obstructs exposure. It is therefore a common practice to use a belt produced by cutting an extruded tubular material to a predetermined width.
On the other hand, the endless belt for the above application should be resistive to heat, highly transparent for ultraviolet rays, and not easy to collect impurities. While a belt made of Teflon will meet such requirements, such a belt is little flexible and cannot be readily provided with an endless configuration by extrusion. The conventional transport belt is therefore expensive and inaccurate. Moreover, a Teflon belt having poor flexibility if apt to shift on the rollers in the axial direction of the latter. Although an arrangement for sensing the shift of the belt and then moving the belt in the axial direction has been proposed, it is not practicable without resorting to a complicated and expensive structure.